1. Field of the Invention
The present invention is directed to a method for scanning an examination subject with a CT (computed tomography) device and to a CT device for implementing the method. The CT device is of the type having an X-ray source that emits an X-ray beam, and a detector system composed of several rows of detector elements on which the beam is incident, the X-ray beam being displaceable relative to a system axis, and the detector signals that are formed in the various projections being fed to a computer, which constitutes images of the examination subject therefrom.
2. Description of the Prior Art
Known radiographic CT devices have an X-ray source that directs a collimated pyramidal X-ray beam through the examination subject, for instance a patient, onto a detector system that is constructed of a two-dimensional array of detector elements, which is completely irradiated by the X-ray source. The X-ray source and, depending on the construction of the radiographic CT device, the detector system as well, are attached on a gantry that rotates around the examination subject. A support table for the examination subject can be moved, relative to the gantry. The longitudinal position from which the X-ray beam penetrates the examination subject and the angle at which the X-ray beam penetrates the examination subject are continuously modified as a consequence of the rotation of the gantry and this relative movement. Every detector element of the detector system that is struck by X-rays produces a signal, which represents a measure of the total transparency of the body of the examination subject for the X-rays emanating from the X-ray source on its path to the detector system. The set of output signals of the detector elements of the detector system that is obtained for a particular angular position of the X-ray source is referred to as a projection. A scan comprises a set of projections which are obtained at different positions of the gantry and/or at different positions of the support table. The radiographic CT device picks up a number of projections during one scan, in order to be able to construct a two-dimensional tomogram of a slice of the examination subject. Several slices can be picked up at once by the detector system that is formed by several detector rows.
Larger volumes of the examination subject are usually picked up by sequential scanning or spiral scanning.
In sequential scanning, the data are picked up during the rotational movement of the gantry, while the examination subject is situated in a fixed position, and so a plane slice is scanned. Between scans of consecutive slices, the examination subject is moved into a new position, in which the next slice can be scanned. This process is continued until all slices that were specified prior to the examination have been scanned.
In spiral scanning the gantry with the X-ray source rotates continuously around the examination subject, while the support table and the gantry are continuously displaced relative to one another along a system axis. The X-ray source thus describes a spiral (helical) path in relation to the examination subject, until the volume that was specified prior to the examination has been scanned. Images of individual slices are then computed from the spiral data obtained in this manner.
In conventional radiographic CT devices, the cross-section of the X-ray beam is set by means of a radiation diaphragm at the source prior to the scanning of an examination subject and is not modified during this scan (German OS 197 21 535). Because of this, regions of the examination subject that are not used for image reconstruction are frequently penetrated by X-rays, particularly in CT devices with a multiline detector system. This is also true of a radiographic CT system as taught in German OS 197 48 891, in which it is possible to modify the slice thickness during the scan according to the body region of the examination subject currently being scanned.